Abstract: There is a phenomenon along China Southeast Coastal Seismic Zone that the earthquakes swarming and seismic tranquility have been alternately occurred since 20th century. According to the distribution of seismic strain relief, there are several results of division between the active episodes and quiet episodes. Using the square-error method among ordered sample sequence clustering analysis on 54 earthquakes occurrence time interval series, and according to the changes of parameters of the error function e.g. the point of inflexion, F value etc., we consider that dividing result of 4 active seismic episodes is more reasonable. Considering another parameter of the earthquake occurrence time interval series-time variation coefficient δ, the calculated results for moderately strong earthquake series (M≥4) along southeast coastal seismic zone since 1943 indicate that earthquake clustering situation has been obvious from then. For each of the 4 active episodes, the accumulated number of earthquakes vs. the earthquake occurrence time basically accorded with the formula N=ct^d. The evolution power exponent d changed from about 0.7 to 0.8, the accumulated number of strong earthquakes (M≥6.0) accorded with the formula too, but its power exponent d is 0.47.

Key words: Seismically active episode, Clustering characteristics, Ordered sample sequence cluster analysis, Active episode time evolution power exponent